The present invention relates to a method of controlling calcium, e.g. calcite in a uranium and/or related values, e.g., molybdenum, manganese, titanium, etc. recovery operation and more particularly relates to a method of controlling calcite build-up in the leach solution of an in situ leach operation by precipitating calcium from the leach solution after leach solution has been produced to the surface.
In a typical in situ leach operation for recovering uranium and/or related metal values, wells are completed into a mineral bearing ore formation and a leach solution is flowed between the walls. The uranium and/or related values are dissolved into the leach solution and are produced therewith to the surface where the pregnant leach solution is treated to recover the desired values.
In many of the known uranium and other mineral bearing formations, a substantial part of the formation matrix is comprised of calcium-based clays (e.g., smectite) and/or limestones. When certain leach solutions (e.g., carbonate and/or bicarbonate solutions) are used in these types of formations, the calcium clays and/or limestones react with the leach solutions to form substantial amounts of calcite (i.e., calcium carbonate) in the leach solutions which, if not properly controlled, will precipitate within the leaching circuit, thereby creating scaling and/or plugging problems throughout the operation.
Certain procedures are known for combating calcite build-up in the leaching circuit. For example, excess carbon dioxide (CO.sub.2) can be added to the leach solution at various points in the leach circuit which aids in keeping the calcite in solution. However, it may be difficult to keep the CO.sub.2 in solution since it easily vaporizes out of the leach solution at the pressures and temperatures normally existing in the leach circuit. Also, special equipment, such as calcite precipitators and/or commercially available water softeners can be used in the surface processing of the leach solution to physically and/or chemically remove calcite. Although complete removal of the calcite from the leach solution is most desirable, in many known leach operations the total removal of calcite from the leach solution with such equipment would be extremely expensive and would seriously affect the overall success of a commercial leach operation. Accordingly, there is normally some calcite still present in the leach solution even after treatment with such equipment, and this calcite can foul the extraction means used for extracting the uranium and/or related values from the leach solution. Also, since the barren leach solution may be used to make fresh leach solution, calcite in barren leach solution may precipitate in the mixing tanks and/or the injection wells.
Further, as is known, certain chemicals commonly called "inhibitors" can be added to a solution to prevent scale build-up due to calcite precipitation. However, the indiscriminate use of such inhibitors in an in situ leaching cycle where the leach solution is to be reused may be detrimental since normal amounts of inhibitors tend to stabilize the calcite in the leach solution, causing the calcite to build up in the recycled leach solution to levels which may threaten the overall leach operation. Also, selective use of inhibitors has been proposed to keep the calcite in solution for a specified time after which it can be precipitated out of solution at designated points in the leach circuit, see U.S. Pat. No. 4,103,963.